1. Field of the Invention
The present invention relates generally to an optical waveguide. In particular, a phosphor coated waveguide for efficient collection and detection of back-scattered electrons in an electron beam apparatus such as a scanning electron microscope is disclosed.
2. Description of Related Art
An electron beam apparatus that incorporates an electron beam microcolumn may be used in electron beam lithography as well as in electron microscopes such as a scanning electron microscope. Electron microscopes are often utilized to image and measure features on semiconductor wafers and can facilitate detection of contaminants. In an electron beam apparatus, a specimen to be examined, such as a semiconductor wafer, is scanned by an electron beam focused onto the specimen. Back-scattered and secondary electrons result from the electron beam impacting the specimen.
Backscattered and secondary electrons may be detected using scintillators. In a scintillator, electrons strike a phosphor coating on a surface of a waveguide and are converted to photons. The phosphor coating is usually deposited on a surface of the waveguide about an axis of the electron beam. The photons generated as a result of the electrons striking the phosphor are collected and directed through the waveguide to an end where an optical detector is placed. The waveguide is generally disposed such that the photons are directed along a length of the waveguide perpendicular to the electron beam axis toward the optical detector. The optical detector such as a photomultiplier tube (PMT) detects the photons that reach the end of the waveguide.
FIG. 1 is a schematic of a conventional electron beam microscope system 20 and FIG. 2 is a top view of a conventional waveguide 30 utilized by the electron beam microscope system of FIG. 1. As shown in FIG. 1, the electron beam system 20 includes an electron beam source 22 that generates and focuses an electron beam 24 through the waveguide 30 onto a specimen 26 to be examined. Back-scattered and secondary electrons 28 result from the electron beam 24 impacting the specimen 26 and are generally directed toward the waveguide 30 and/or a phosphor coated region 44 of the waveguide 30. The waveguide is typically made of glass or plastic.
As shown in FIGS. 1 and 2, the waveguide 30 includes two side faces 32 as well as angled faces 34 extending between a top and a bottom face 36, 38, respectively. An optical detector (not shown) is located at an end 40 of the waveguide 30. The waveguide defines a hole 42 about an axis of the electron beam through which the electron beam passes. In addition, the phosphor coated region 44 of the waveguide is typically an annular phosphor coating on portions of the angled faces 34 about the hole 42.
As noted above, back-scattered and secondary electrons strike the phosphor coating 44 and are converted to photons that are ideally directed by the waveguide 30 toward the waveguide end 40 for detection by the optical detector. The angled faces 34 tend to reflect photons toward the end 40 either directly or off the side, top and/or bottom faces 32, 36, 38, respectively.
However, conventional electron beam microscope systems such as the one shown and described with reference to FIGS. 1 and 2 typically have low collection efficiency, thereby limiting the speed at which the conventional systems can be operated. As is well known in optics, an angle of incidence xcex8i, i.e., measured relative to the normal of an interface or surface that the photons strike, greater than or equal to the critical angle achieves total internal reflection, i.e., no refraction. In contrast, at least a portion of the photons that strike a surface at an angle less than the critical angle is transmitted through the waveguide material, i.e., refracted. Refraction of the photons decreases the collection efficiency in that the refracted photons do not reach and thus are not detected by the detector.
The critical angle depends upon the relative refractive indexes of the two different materials through which light travels. Because electron beam microscope systems operate in vacuum (nvacuum=1), the critical angle is given by Arc sin (1/n) where n is the refractive index of the waveguide material.
In addition, the photon collection efficiency may not be homogeneous in that the collection efficiency may be dependent upon where the electron strikes the phosphor. In the electron beam microscope system shown in FIGS. 1 and 2, electrons that strike the phosphor on the right side of the hole are more efficiently collected than those that strike the left side of the hole. The collection inhomogeneity leads to a reduced contrast depending upon how the electrons scatter from the specimen.
As advances in semiconductor fabrication technologies have enabled fabrication of smaller and smaller integrated circuits, it has become increasingly important to accurately, efficiently, and effectively detect contamination on the semiconductor wafers in a time-efficient manner. Thus, it is desirable to provide a waveguide that has an improved collection efficiency by providing a waveguide that results in greater portion of photons being detected by the optical detector. It is also desirable to provide a waveguide that has an improved collection efficiency homogeneity. It is further desirable to limit the size of the waveguide, e.g., to approximately 1.5 mm in thickness and/or approximately 6 mm in width, depending upon its application.
A phosphor coated waveguide for efficient collection and detection of back-scattered electrons in an electron beam apparatus such as a scanning electron microscope is disclosed. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
According to one preferred embodiment, a waveguide for use in an electron microscope generally comprises a first waveguide portion having opposing first and second faces defining a beveled hole therebetween to allow an electron beam to pass therethrough, the beveled hole decreasing in cross-sectional size from the first to the second face. The second face has a phosphor coating around the beveled hole. The beveled hole includes a beveled portion and optionally a straight portion. The beveled portion defines a beveled surface preferably coated with a reflective material. Generally, the beveled surface may be at an angle between approximately 35xc2x0 and 55xc2x0, and more preferably at 45xc2x0, relative to the first face. The waveguide optionally includes a second waveguide portion having opposing first and second ends, the first end being coupled to the first waveguide portion and the second end being larger than the first end and adapted to be coupled to an optical detector.
In another embodiment, a waveguide for use in an electron microscope generally comprises a first waveguide portion having opposing first and second faces defining a hole therebetween to allow an electron beam to pass therethrough, a phosphor coating on the second face disposed about the hole, and a second waveguide portion having opposing first and second ends, the first end being adapted to be coupled to the first waveguide portion and the second end being larger than the first end and adapted to be coupled to an optical detector.
The second waveguide portion has first and second sides that preferably taper at a taper angle relative to the first and second faces of the first waveguide portion, respectively. The taper angle is generally between approximately 7xc2x0 and 15xc2x0, and more preferably approximately 10xc2x0. Alternatively the second side is non-tapered while the taper for the first side is increased to approximately 15xc2x0 to 20xc2x0.
With regard to any of the waveguide embodiments, the first waveguide portion may further comprise opposing first and second ends, the second end being disposed toward an optical detector and the first end having a plurality of adjoining angled faces extending between the first and second faces of the first waveguide portion. The angled faces may include two or three angled faces forming an angle of approximately 90xc2x0 or 135xc2x0, respectively, therebetween. The angled faces may have a reflective coating thereon.
In yet another alternative embodiment, the waveguide may also include a cylindrical light guide adapted to be coupled to the first or second waveguide portion and an optical detector. The cylindrical light guide generally comprises a face adapted to be coupled to the first or second waveguide portion for receiving light therefrom, an angled face having a reflective coating thereon and disposed at approximately 45xc2x0 relative to an axis of the cylindrical light guide for reflecting light received by the face, and a cylindrical section through which light reflected from the angled face is guided to the optical detector.
These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures which illustrate by way of example the principles of the present invention.